Flexible substrates offer the promise of cheaper devices using roll-to-roll processing, and the potential to make thinner, lighter, more flexible and durable displays. However, the technology, equipment, and processes required for roll-to-roll processing of high quality displays are not yet fully developed. Since panel makers have already heavily invested in toolsets to process large sheets of glass, laminating a flexible substrate to a carrier and making display devices by a sheet-to-sheet processing offers a shorter term solution to develop the value proposition of thinner, lighter, and more flexible displays. Displays have been demonstrated on polymer sheets for example polyethylene naphthalate (PEN) where the device fabrication was sheet to sheet with the PEN laminated to a glass carrier. The upper temperature limit of the PEN limits the device quality and process that can be used. In addition, the high permeability of the polymer substrate leads to environmental degradation of OLED devices where a near hermetic package is required. Thin film encapsulation offers the promise to overcome this limitation, but it has not yet been demonstrated to offer acceptable yields at large volumes.
In a similar manner, display devices can be manufactured using a glass carrier laminated to one or more thin glass substrates. It is anticipated that the low permeability and improved temperature and chemical resistance of the thin glass will enable higher performance longer lifetime flexible displays.
However, the thermal, vacuum, solvent and acidic, and ultrasonic, Flat Panel Display (FPD) processes require a robust bond for thin glass bound to a carrier. FPD processes typically involve vacuum deposition (sputtering metals, transparent conductive oxides and oxide semiconductors, Chemical Vapor Deposition (CVD) deposition of amorphous silicon, silicon nitride, and silicon dioxide, and dry etching of metals and insulators), thermal processes (including ˜300-400° C. CVD deposition, up to 600° C. p-Si crystallization, 350-450° C. oxide semiconductor annealing, up to 650° C. dopant annealing, and ˜200-350° C. contact annealing), acidic etching (metal etch, oxide semiconductor etch), solvent exposure (stripping photoresist, deposition of polymer encapsulation), and ultrasonic exposure (in solvent stripping of photoresist and aqueous cleaning, typically in alkaline solutions).
Adhesive wafer bonding has been widely used in Micromechanical Systems (MEMS) and semiconductor processing for back end steps where processes are less harsh. Commercial adhesives by Brewer Science and Henkel are typically thick polymer adhesive layers, 5-200 microns thick. The large thickness of these layers creates the potential for large amounts of volatiles, trapped solvents, and adsorbed species to contaminate FPD processes. These materials thermally decompose and outgas above ˜250° C. The materials also may cause contamination in downstream steps by acting as a sink for gases, solvents and acids which can outgas in subsequent processes.
U.S. Provisional Application Ser. No. 61/596,727 filed on Feb. 8, 2012, entitled Processing Flexible Glass with a Carrier (hereinafter U.S. '727) discloses that the concepts therein involve bonding a thin sheet, for example, a flexible glass sheet, to a carrier initially by van der Waals forces, then increasing the bond strength in certain regions while retaining the ability to remove portions of the thin sheet after processing the thin sheet/carrier to form devices (for example, electronic or display devices, components of electronic or display devices, organic light emitting device (OLED) materials, photo-voltaic (PV) structures, or thin film transistors), thereon. At least a portion of the thin glass is bonded to a carrier such that there is prevented device process fluids from entering between the thin sheet and carrier, whereby there is reduced the chance of contaminating downstream processes, i.e., the bonded seal portion between the thin sheet and carrier is hermetic, and in some preferred embodiments, this seal encompasses the outside of the article thereby preventing liquid or gas intrusion into or out of any region of the sealed article.
U.S. '727 goes on to disclose that in low temperature polysilicon (LTPS) (low temperature compared to solid phase crystallization processing which can be up to about 750° C.) device fabrication processes, temperatures approaching 600° C. or greater, vacuum, and wet etch environments may be used. These conditions limit the materials that may be used, and place high demands on the carrier/thin sheet. Accordingly, what is desired is a carrier approach that utilizes the existing capital infrastructure of the manufacturers, enables processing of thin glass, i.e., glass having a thickness ≤0.3 mm thick, without contamination or loss of bond strength between the thin glass and carrier at higher processing temperatures, and wherein the thin glass de-bonds easily from the carrier at the end of the process.
One commercial advantage to the approach disclosed in U.S. '727 is that, as noted in U.S. '727, manufacturers will be able to utilize their existing capital investment in processing equipment while gaining the advantages of the thin glass sheets for PV, OLED, LCDs and patterned Thin Film Transistor (TFT) electronics, for example. Additionally, that approach enables process flexibility, including: that for cleaning and surface preparation of the thin glass sheet and carrier to facilitate bonding; that for strengthening the bond between the thin sheet and carrier at the bonded area; that for maintaining releasability of the thin sheet from the carrier at the non-bonded (or reduced/low-strength bond) area; and that for cutting the thin sheets to facilitate extraction from the carrier.
In the glass-to-glass bonding process, the glass surfaces are cleaned to remove all metal, organic and particulate residues, and to leave a mostly silanol terminated surface. The glass surfaces are first brought into intimate contact where van der Waals and/or Hydrogen-bonding forces pull them together. With heat and optionally pressure, the surface silanol groups condense to form strong covalent Si—O—Si bonds across the interface, permanently fusing the glass pieces. Metal, organic and particulate residue will prevent bonding by obscuring the surface preventing the intimate contact required for bonding. A high silanol surface concentration is also required to form a strong bond as the number of bonds per unit area will be determined by the probability of two silanol species on opposing surfaces reacting to condense out water. Zhuravlel has reported the average number of hydroxyls per nm2 for well hydrated silica as 4.6 to 4.9. Zhuravlel, L. T., The Surface Chemistry of Amorphous Silika, Zhuravlev Model, Colloids and Surfaces A: Physiochemical Engineering Aspects 173 (2000) 1-38. In U.S. '727, a non-bonding region is formed within a bonded periphery, and the primary manner described for forming such non-bonding area is increasing surface roughness. An average surface roughness of greater than 2 nm Ra can prevent glass to glass bonds forming during the elevated temperature of the bonding process. In U.S. Provisional Patent Application Ser. No. 61/736,880, filed on Dec. 13, 2012 by the same inventors and entitled Facilitated Processing for Controlling Bonding Between Sheet and Carrier (hereinafter U.S. '880), a controlled bonding area is formed by controlling the van der Waals and/or hydrogen bonding between a carrier and a thin glass sheet, but a covalent bonding area is still used as well. Thus, although the articles and methods for processing thin sheets with carriers in U.S. '727 and U.S. '880 are able to withstand the harsh environments of FPD processing, undesirably for some applications, reuse of the carrier is prevented by the strong covalent bond between thin glass and glass carrier in the bonding region that is bonded by covalent, for example Si—O—Si, bonding with adhesive force ˜1000-2000 mJ/m2, on the order of the fracture strength of the glass. Prying or peeling cannot be used to separate the covalently bonded portion of the thin glass from the carrier and, thus, the entire thin sheet cannot be removed from the carrier. Instead, the non-bonded areas with the devices thereon are scribed and extracted leaving a bonded periphery of the thin glass sheet on the carrier.